Doing little other than pulsating gently all day, it turns out this is enough work for the jellyfish Cassiopea, the upside-down jellyfish, to want a good night’s rest. The recent discovery out of Caltech laboratories that these simple creatures need to “get some shut eye” when they don’t even have eyes, or even a brain, is an important new finding that has far reaching implications about the nature of sleep.

Multiple Cassiopea jellyfish on the bottom of a tank.(Photo Credit: Caltech)

“It may not seem surprising that jellyfish sleep—after all, mammals sleep, and other invertebrates such as worms and fruit flies sleep,” says study co-author Ravi Nath in a press release.“But jellyfish are the most evolutionarily ancient animals known to sleep. This finding opens up many more questions: Is sleep the property of neurons? And perhaps a more far-fetched question: Do plants sleep?”

Despite decades of study, sleep still remains one of the great mysterious frontiers of science. Indeed, William Dement, a co-discoverer of REM sleep, was famously quoted responding to the question of why we need sleep.

“As far as I know,” he answers in the National Geographic article Secrets of Sleep, “the only reason we need to sleep that is really, really solid is because we get sleepy.”

Why the primitive jellyfish Cassiopea needs sleep is just as mysterious as why people need sleep — if not more so considering the ancient organism lacks a brain. And despite this lack of a brain, or even a traditional central nervous system, the existence of a sleep state reminds us of the similarities between them and higher order animals such as humans.

“When humans sleep, we are inactive, we often can sleep through noises or other disturbances which we might otherwise react to if we were awake, and we’re likely to fall asleep during the day if we don’t get enough sleep,” says study co-author Claire Bedbrook. “We might seem extremely different from jellyfish, but we both exhibit a similar sleep state.”

To show this, the scientists gently “poked” the jellyfish with a pulse of water during the night when they slowed their movements and entered what was suspected to be their sleep state. The jellyfish showed signs of waking up when stirred. What’s more, when frequently awakened in this manner over the course of a night, they were more likely to fall into inactivity during the day, as if drifting off into naps due to the lost rest.

Another method used to determine that Cassiopea have a sleep state involved lifting them off the floor of the tank. Normally, while awake, the jellyfish immediately swim back to the bottom if removed. But, if the scientists gently lifted them off the floor while they slept, the jellyfish would float in place for a short period before waking up, reorienting themselves, and heading back to the bottom.

Yet another experiment saw the scientists giving them compounds that induced sleep in other animals, including melatonin, which many people take to help get to bed.

“We found that these compounds did affect jellyfish sleep in the predicted ways, suggesting that their underlying sleep mechanism is similar to those of other organisms—including humans,” says another of the study’s co-authors Michael Abrams.

With the scientific criteria of sleep having been exhibited by an ancient jellyfish species, the researchers say it now seems that sleep is a behavior that arose “early in the metazoan lineage.” That is to say, hundreds of millions of years ago, before brains or central nervous systems evolved, life got sleepy.

As with many groundbreaking studies, especially in sleep science, the research raises more questions than answers and is expected to spur further research.

The findings on Cassiopea were the product of collaboration between a group of Caltech scientists with expertise across several disciplines. The research was published in the September 21 issue of Current Biology.